1. Field of Invention
The current invention relates to feedhorns for receiving and/or transmitting electromagnetic radiation, and more particularly to smooth-walled feedhorns for receiving and/or transmitting electromagnetic radiation.
2. Discussion of Related Art
Many precision microwave applications, including those associated with radio astronomy, require feedhorns with symmetric E- and H-plane beam patterns that possess low sidelobes and cross-polarization control. A common approach to achieving these goals is a “scalar” feed, which has a beam response that is independent of azimuthal angle. Corrugated feeds (P. Clarricoats and A. Olver, Corrugated Horns for Microwave Antennas. London, U.K.: Peregrinus, 1984) approximate this idealization by providing the appropriate boundary conditions for the HE11 hybrid mode at the feed aperture. Corrugated feedhorns require high-precision grooves in the walls of the feedhorns, often to a within a small fraction of a wavelength (e.g., ˜0.0022λc where λc is the cutoff wavelength of the input guide section). In addition, the manufacturing by direct machining of each groove can leave small burrs in the grooves that can adversely affect the properties of the feedhorn, thus requiring further labor-intensive inspection and correction. Alternatively, chemically electroformed corrugated feed horns require the use of a precision mandrel for each assembly which is destroyed in the fabrication process. Consequently, feedhorns that have corrugated walls are expensive and labor-intensive to produce.
Alternatively, an approximation to a scalar feed can be obtained with a multimode feed design. One such “dual-mode” horn is the Potter horn (P. Potter, “A new horn antenna with suppressed sidelobes and equal beamwidths,” Microwave Journal, pp. 71-78, June 1963). In this implementation, an appropriate admixture of TM11 is generated from the initial TE11 mode using a concentric step discontinuity in the waveguide. The two modes are then phased to achieve the proper field distribution at the feed aperture using a length of waveguide. The length of the phasing section limits the bandwidth due to the dispersion between the modes. Lier (E. Lier, “Cross polarization from dual mode horn antennas,” IEEE Transactions on Antennas and Propagation, vol. 34, no. 1, pp. 106-110, 1986) has reviewed the cross-polarization properties of dual-mode horn antennas for selected geometries. Others have produced variations on this basic design concept (R. Turrin, “Dual mode small-aperture antennas,” IEEE Transactions on Antennas and Propagation, vol. 15, no. 2, pp. 307-308, 1967; G. Ediss, “Technical memorandum. dual-mode horns at millimeter and submillimeter wavelengths,” IEE Proceedings H Microwaves Antennas and Propagation, vol. 132, no. 3, pp. 215-218, 1985). Improvements in the bandwidth have been realized by decreasing the phase difference between the two modes by 2π (H. Pickett, J. Hardy, and J. Farhoomand, “Characterization of a dual-mode horn for submillimeter wavelengths (short papers),” IEEE Transactions on Microwave Theory and Techniques, vol. 32, no. 8, pp. 936-937, 1984; S. Skobelev, B.-J. Ku, A. Shishlov, and D.-S. Ahn, “Optimum geometry and performance of a dual-mode horn modification,” IEEE Antennas and Propagation Magazine, vol. 43, no. 1, pp. 90-93, 2001).
To increase the bandwidth, it is possible to add multiple concentric step continuities with the appropriate modal phasing (T. S. Bird, “A multibeam feed for the parker radio-telescope,” IEEE Antennas & Propagation Symposium, pp. 966-969, 1994; S. M. Tun and P. Foster, “Computer optimised wideband dual-mode horn,” Electronics Letters, vol. 38, no. 15, pp. 768-769, 2001). A variation on this technique is to use several distinct linear tapers to generate the proper modal content and phasing (G. Yassin, P. Kittara, A. Jiralucksanawong, S. Wangsuya, J. Leech, and M. Jones, “A high performance horn for large format focal plane arrays,” 18th International Symposium on Space Terahertz Technology, pp. 1-12, April 2008; P. Kittara, A. Jiralucksanawong, G. Yassin, S. Wangsuya, and J. Leech, “The design of potter horns for THz applications using a genetic algorithm,” International Journal of Infrared and Millimeter Waves, vol. 28, pp. 1103-1114, 2007). Operational bandwidths of 15-20% have been reported using such techniques. A related class of devices is realized by allowing the feedhorn profile to vary smoothly rather than in discrete steps. Examples of such smooth-walled feedhorns with about 15% fractional bandwidths have been reported (G. Granet, G. L. James, R. Bolton, and G. Moorey, “A smooth-walled spline-profile horn as an alternative to the corrugated horn for wide band millimeter-wave applications,” IEEE Transactions on Antennas and Propagation, vol. 52, no. 3, pp. 848-854, 2004; J. M. Neilson, “An improved multimode horn for Gaussian mode generation at millimeter and submillimeter wavelengths,” IEEE Transactions on Antennas and Propagation, vol. 50, no. 8, pp. 1077-1081, 2002). However, there remains a need for improved smooth-walled feedhorns, for example, smooth-walled feedhorns that have greater than a 15% bandwidth with low cross-polarization response.